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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="intrusive.map_multimap"></a><a class="link" href="map_multimap.html" title="Map and multimap-like interface for associative containers">Map and multimap-like interface
    for associative containers</a>
</h2></div></div></div>
<p>
      Implementing map-like intrusive containers is not a trivial task as STL's
      <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code> and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">multimap</span></code>
      containers store copies of a <code class="computeroutput"><span class="identifier">value_type</span></code>
      which is defined as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">key_type</span><span class="special">,</span> <span class="identifier">mapped_type</span><span class="special">&gt;</span></code>. In order to reproduce this interface in
      <span class="bold"><strong>Boost.Intrusive</strong></span> it shall require that objects
      stored in the intrusive containers contain that <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code> member
      with <code class="computeroutput"><span class="identifier">pair</span><span class="special">.</span><span class="identifier">first</span></code> hardcoded as the key part and <code class="computeroutput"><span class="identifier">pair</span><span class="special">.</span><span class="identifier">second</span></code>
      hardcoded as the <code class="computeroutput"><span class="identifier">mapped_type</span></code>,
      which is limiting and also not very useful in practice. Any intrusive associative
      container can be used like a map using <a class="link" href="advanced_lookups_insertions.html" title="Advanced lookup and insertion functions for associative containers">advanced
      lookup and insertions</a> and the user can change the key type in each lookup/insertion
      check call.
    </p>
<p>
      On the other hand, in many cases containers are indexed by a well-known key
      type, and the user is forced to write some repetitive code using advanced lookup
      and insertions. <span class="bold"><strong>Boost.Intrusive</strong></span> associative
      containers offer an alternative to build a useful map-like lookup interfaces
      without forcing users to define <code class="computeroutput"><span class="identifier">value_type</span></code>s
      containing <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>-like classes. The option is called
      <code class="computeroutput"><a class="link" href="../boost/intrusive/key_of_value.html" title="Struct template key_of_value">boost::intrusive::key_of_value</a></code>.
    </p>
<p>
      If a user specifies that option when defining a <code class="computeroutput"><span class="identifier">set</span><span class="special">/</span><span class="identifier">multiset</span></code>
      intrusive container, it specifies a function object that will tell the container
      which is the type of the <span class="emphasis"><em>key</em></span> that <code class="computeroutput"><span class="identifier">value_type</span></code>
      holds and how to obtain it. This function object must be:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          Lightweight to copy.
        </li>
<li class="listitem">
          Default constructible (when the container constructor overload requires
          it).
        </li>
<li class="listitem">
          It shall define:
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem">
                A <code class="computeroutput"><span class="identifier">type</span></code> member that
                defines the type of the key
              </li>
<li class="listitem">
                A member function that returns the key derived a <code class="computeroutput"><span class="identifier">value_type</span></code>,
                either by value or by const-reference.
              </li>
</ul></div>
        </li>
</ul></div>
<p>
      Let's see an example of how a set can be configured as a map indexed by an
      integer stored in the <code class="computeroutput"><span class="identifier">value_type</span></code>:
    </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="keyword">static_assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">is_same</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">set</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">unordered_set</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cassert</span><span class="special">&gt;</span>

<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>

<span class="keyword">class</span> <span class="identifier">MyClass</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">set_base_hook</span><span class="special">&lt;&gt;</span>
              <span class="special">,</span> <span class="keyword">public</span> <span class="identifier">unordered_set_base_hook</span><span class="special">&lt;&gt;</span>
<span class="special">{</span>
   <span class="keyword">public</span><span class="special">:</span>
   <span class="keyword">int</span> <span class="identifier">first</span><span class="special">;</span>
   <span class="keyword">explicit</span> <span class="identifier">MyClass</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">first</span><span class="special">(</span><span class="identifier">i</span><span class="special">){}</span>
<span class="special">};</span>

<span class="comment">//key_of_value function object, must:</span>
<span class="comment">//- be default constructible if the container constructor requires it</span>
<span class="comment">//- define the key type using "type"</span>
<span class="comment">//- define an operator() taking "const value_type&amp;" and</span>
<span class="comment">//    returning "type" or "const type &amp;"</span>
<span class="keyword">struct</span> <span class="identifier">first_int_is_key</span>
<span class="special">{</span>
   <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">type</span><span class="special">;</span>

   <span class="keyword">const</span> <span class="identifier">type</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">MyClass</span><span class="special">&amp;</span> <span class="identifier">v</span><span class="special">)</span> <span class="keyword">const</span>
   <span class="special">{</span>  <span class="keyword">return</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">first</span><span class="special">;</span>  <span class="special">}</span>
<span class="special">};</span>

<span class="comment">//Define omap like ordered and unordered classes </span>
<span class="keyword">typedef</span> <span class="identifier">set</span><span class="special">&lt;</span> <span class="identifier">MyClass</span><span class="special">,</span> <span class="identifier">key_of_value</span><span class="special">&lt;</span><span class="identifier">first_int_is_key</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">OrderedMap</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">unordered_set</span><span class="special">&lt;</span> <span class="identifier">MyClass</span><span class="special">,</span> <span class="identifier">key_of_value</span><span class="special">&lt;</span><span class="identifier">first_int_is_key</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">UnorderedMap</span><span class="special">;</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
   <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span>  <span class="identifier">OrderedMap</span><span class="special">::</span><span class="identifier">key_type</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">));</span>
   <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><span class="identifier">UnorderedMap</span><span class="special">::</span><span class="identifier">key_type</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">));</span>

   <span class="comment">//Create several MyClass objects, each one with a different value</span>
   <span class="comment">//and insert them into the omap</span>
   <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">MyClass</span><span class="special">&gt;</span> <span class="identifier">values</span><span class="special">;</span>
   <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="number">100</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>  <span class="identifier">values</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">MyClass</span><span class="special">(</span><span class="identifier">i</span><span class="special">));</span>

   <span class="comment">//Create ordered/unordered maps and insert values</span>
   <span class="identifier">OrderedMap</span>   <span class="identifier">omap</span><span class="special">(</span><span class="identifier">values</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">values</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
   <span class="identifier">UnorderedMap</span><span class="special">::</span><span class="identifier">bucket_type</span> <span class="identifier">buckets</span><span class="special">[</span><span class="number">100</span><span class="special">];</span>
   <span class="identifier">UnorderedMap</span> <span class="identifier">umap</span><span class="special">(</span><span class="identifier">values</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">values</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">UnorderedMap</span><span class="special">::</span><span class="identifier">bucket_traits</span><span class="special">(</span><span class="identifier">buckets</span><span class="special">,</span> <span class="number">100</span><span class="special">));</span>

   <span class="comment">//Test each element using the key_type (int)</span>
   <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">!=</span> <span class="number">100</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">){</span>
      <span class="identifier">assert</span><span class="special">(</span><span class="identifier">omap</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">!=</span> <span class="identifier">omap</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
      <span class="identifier">assert</span><span class="special">(</span><span class="identifier">umap</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">!=</span> <span class="identifier">umap</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
      <span class="identifier">assert</span><span class="special">(</span><span class="identifier">omap</span><span class="special">.</span><span class="identifier">lower_bound</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">!=</span> <span class="identifier">omap</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
      <span class="identifier">assert</span><span class="special">(++</span><span class="identifier">omap</span><span class="special">.</span><span class="identifier">lower_bound</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">omap</span><span class="special">.</span><span class="identifier">upper_bound</span><span class="special">(</span><span class="identifier">i</span><span class="special">));</span>
      <span class="identifier">assert</span><span class="special">(</span><span class="identifier">omap</span><span class="special">.</span><span class="identifier">equal_range</span><span class="special">(</span><span class="identifier">i</span><span class="special">).</span><span class="identifier">first</span> <span class="special">!=</span> <span class="identifier">omap</span><span class="special">.</span><span class="identifier">equal_range</span><span class="special">(</span><span class="identifier">i</span><span class="special">).</span><span class="identifier">second</span><span class="special">);</span>
      <span class="identifier">assert</span><span class="special">(</span><span class="identifier">umap</span><span class="special">.</span><span class="identifier">equal_range</span><span class="special">(</span><span class="identifier">i</span><span class="special">).</span><span class="identifier">first</span> <span class="special">!=</span> <span class="identifier">umap</span><span class="special">.</span><span class="identifier">equal_range</span><span class="special">(</span><span class="identifier">i</span><span class="special">).</span><span class="identifier">second</span><span class="special">);</span>
   <span class="special">}</span>

   <span class="comment">//Count and erase by key</span>
   <span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">!=</span> <span class="number">100</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">){</span>
      <span class="identifier">assert</span><span class="special">(</span><span class="number">1</span> <span class="special">==</span> <span class="identifier">omap</span><span class="special">.</span><span class="identifier">count</span><span class="special">(</span><span class="identifier">i</span><span class="special">));</span>
      <span class="identifier">assert</span><span class="special">(</span><span class="number">1</span> <span class="special">==</span> <span class="identifier">umap</span><span class="special">.</span><span class="identifier">count</span><span class="special">(</span><span class="identifier">i</span><span class="special">));</span>
      <span class="identifier">assert</span><span class="special">(</span><span class="number">1</span> <span class="special">==</span> <span class="identifier">omap</span><span class="special">.</span><span class="identifier">erase</span><span class="special">(</span><span class="identifier">i</span><span class="special">));</span>
      <span class="identifier">assert</span><span class="special">(</span><span class="number">1</span> <span class="special">==</span> <span class="identifier">umap</span><span class="special">.</span><span class="identifier">erase</span><span class="special">(</span><span class="identifier">i</span><span class="special">));</span>
   <span class="special">}</span>
   <span class="identifier">assert</span><span class="special">(</span><span class="identifier">omap</span><span class="special">.</span><span class="identifier">empty</span><span class="special">());</span>
   <span class="identifier">assert</span><span class="special">(</span><span class="identifier">umap</span><span class="special">.</span><span class="identifier">empty</span><span class="special">());</span>

   <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
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<td align="right"><div class="copyright-footer">Copyright © 2005 Olaf Krzikalla<br>Copyright © 2006-2015 Ion Gaztanaga<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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